What is a multimodal message and how do you pass images or file data inside a message content block?

A multimodal message leverages the content blocks structure to combine different modalities, such as text and images, into a single message. This is essential for vision-capable LLMs. The most common use case is passing images, which can be done by providing a base64-encoded image string or a publicly accessible image URL within an image_url content block. The core concept is the ContentBlock — a dictionary with a type key that dictates the structure of the rest of the block [citation:6].